1. Field of the Invention
The invention relates generally to measuring tension in mooring chains in marine applications. More particularly, the invention relates to measuring the tension in anchor chains for use in mooring FPSO vessels.
2. Description of the Prior Art
Floating, production, storage, and offloading (FPSO) vessels include a mooring system that requires measurement of tension in mooring chains for proper installation and for management of loads during operation. FPSOs are often subject to harsh environments and have long service lives requiring that tension measurement components of the mooring system be robust. The design lifetime of an FPSO and its measuring system may exceed 20 years.
Prior art methods for measuring loads in mooring chains have included placing strain gages on the housing of a chainstopper. This arrangement is disadvantageous, because the strain gages are often mounted in an exposed location, subjecting the strain gages to harsh environments. Furthermore, strain gages in this arrangement are difficult to replace, and to recalibrate after replacement.
Alternatively, load cells have been placed directly on a chain link to measure load in the chain when mooring an offshore structure such as an offshore vessel. U.S. Pat. No. 5,845,893 discloses an extensiometer mounted on a latch housing to measure chain force in an anchor chain when it is held by a latch mechanism.
Another prior art method for measuring a load in a mooring chain places a non-contact sensor into the interior of a trunnion block to measure the deflection of the upper and lower trunnions. U.S. Pat. No. 6,925,890 discloses the placement of a non-contact sensor on a trunnion block.
3. Identification of Objects of the Invention
A primary object of the invention is to provide a system for measuring tension in mooring chains that overcomes the disadvantages of the prior art.
Another object of the invention is to provide a system and method for measuring tension in marine mooring chains that achieves reduced maintenance and increased reliability;
Another object of the invention is to provide a system and method for measuring tension in mooring chains that places a tension measuring instrument directly in the load path of the mooring chain;
Still another object of the invention is to provide a system and method for measuring tension in mooring chains with a tension measuring instrument that can be more easily calibrated off-site, prior to installation on the mooring chain.